The closest prior art of which the inventor is aware is the U.S. Pat. No. 4,830,345 (Shih L. Mar, "spring-loaded seat assembly", Ser. No. 122,441, filed Nov. 19, 1987). The spring-loaded seat assembly disclosed is a cushion having two circular seat pieces with a plurality of coil spring members connecting the two seat pieces. The upper circular seat piece is capable of vibrating and oscillating up and down for exercising purposes. The coil spring members vibrate and oscillate freely. Since the elasticity coefficient and the oscillation amplitude of the spring members are very large, the seat assembly may cause injury to old or very young people. In addition, the spring members are apt to be biased and distorted, which may further cause injury to the users, especially to the waist portions of the users.
The present invention has arisen to mitigate and/or obviate the afore described disadvantages of the conventional spring-loaded seat assembly.